1. Field of the Invention
This invention relates to printing presses, and more particularly to web-fed printing presses having a folding station appended thereto for cutting the printed paper web into sections and folding the successive web sections in the middle into the form of signatures. Still more particularly, the invention deals with a jaw cylinder at the folding station, the jaw cylinder having sets of fixed and movable jaws arranged at circumferential spacings thereon for folding the web sections as they are thrust into the creasing jaw cavities by folding blades on a folding cylinder which also is provided at the folding station.
2. Description of the Prior Art
The folding station of a web-fed printing press usually has a cutter cylinder in addition to the noted folding cylinder and jaw cylinder. All these cylinders are in constant rotation during the progress of printing. The printed web of paper is first wrapped around part of the folding cylinder and, while being done so, cut into successive sections by cutting blades on the cutter cylinder which is held against the folding cylinder via the web. The folding cylinder is equipped with elongate folding blades each extending parallel to the folding cylinder axis and arranged at circumferential spacings thereon. Each folding blade is movable radially of the folding cylinder.
Pushed off the surface of the folding cylinder by one of the folding blades, each web section has its midpart inserted in one of the elongate jaw cavities which are cut in the surface of the jaw cylinder at circumferential spacings. The midpart of the web section that has been pushed into the jaw cavity is therein engaged, together with the folding blade, between the fixed and the movable jaw as the movable jaw is closed against the fixed jaw, and thereby folded along the centerline of the web section. The web section is subsequently carried away from the surface of the folding cylinder by the jaw cylinder as these cylinders rotate in opposite directions. The folding blade withdraws from between the fixed and movable jaws just before the web section is folded, usually behaving, in so doing, as if prying open the jaws.
Difficulties have been experienced in folding the web sections between the fixed and movable jaws because they are not necessarily of the same thickness. In practice, two or more printed webs may be superposed one upon another before being cut into sections. The movable jaws were therefore sprung heavily toward the fixed jaws to engage the web sections of variable thickness against the risk of accidental disengagement and to fill up the spaces created upon withdrawal of the folding blades.
The hard springing of the movable jaws is objectionable, however, because the folding blades behave as aforesaid as if prying open the jaws when withdrawing therefrom. Rubbed by the folding blades withdrawing from between the jaws against the forces of the heavy springs, the web sections were easy to be smeared by ink offset between their contacting surfaces, to the serious degradation of the quality of the printings.
Suggestions were made to prevent ink offset between the contacting surfaces of the web sections. Japanese Unexamined Patent Publication No. 54-63904 teaches the provision of strips of elastomer or like material on the movable jaws for elastically pressing the web sections against the fixed jaws. According to Japanese Unexamined Utility Publication No. 60-193365, on the other hand, spring-loaded pins are slidably mounted to the movable jaws. Each folding blade is recessed in places to permit the spring-loaded pins to engage and press the whole web section against the fixed jaw.
An evaluation of these prior art contrivances demands a brief study of dynamics involved in folding web sections by the cooperation of folding cylinder and jaw cylinder. Riding on the jaw cylinder after having its midpart caught between one pair of fixed and movable jaws, each web section is subjected to centrifugal forces proportional to the square of the angular velocity of the jaw cylinder, the diameter of the jaw cylinder, and the mass of the web section itself. Furthermore, when the web section is pulled off the folding cylinder after having its midpart captured by the jaws, the leading half of the web section must travel in sliding engagement with the folding cylinder, thereby to be subjected to an inertial force tending to pull the web section out of engagement with the jaws.
The current trends in rotary printing presses, particularly those for newspaper production, are toward higher printing speeds and more webs placed one upon another before being jointly cut and folded into signatures. The web sections that have been caught by the creasing jaws on the jaw cylinder are being subjected to more and more centrifugal and inertial forces. The web sections must be gripped so firmly as to prevent accidental disengagement in the face of such ever-increasing centrifugal and inertial forces. However, too strong gripping of the web sections is undesirable because of the greater risk of ink transfer between their contacting surfaces.
Let us now reconsider the prior art devices cited above. An objection to Japanese Unexamined Patent Publication No. 54-63904 is that the elastic strips on the movable jaws press the web sections too hard against the fixed jaws because of the greater force urging the movable jaws toward the fixed jaws, often causing ink offset at these parts of the web sections. Ink offset was even easier to occur at the spring-loaded pins taught by Japanese Unexamined Utility Model Publication No. 60-193365.
The present invention seeks, in connection with the folding station of a rotary printing press, to engage and fold web sections between fixed and movable jaws on a jaw cylinder so firmly as to preclude accidental disengagement in the face of substantive centrifugal and inertial forces exerted thereon, but with little or no likelihood of ink transfer between the contacting surfaces of the web sections.
Briefly stated in its perhaps broadest aspect, the present invention concerns, in a folding station of a web-fed printing press where a printed paper web is cut into sections by a cutter cylinder while traveling on a folding cylinder, a jaw cylinder for folding each web section into the form of a signature in cooperation with the folding cylinder. The jaw cylinder has cut in its surface one or more jaw cavities each extending parallel to the jaw cylinder axis and all arranged at constant circumferential spacings. Each jaw cavity has mounted therein a movable jaw movable toward and away from a fixed jaw in order to engage and fold each web section as its midpart is inserted in the jaw cavity by a folding blade on the folding cylinder. A less spacing exists between those parts of the fixed and the movable jaw which are to engage the pair of margins on both sides of the image area of each web section than between those parts of the fixed and the movable jaw which are to engage the image area.
It is thus seen that each web section is prevented from accidental disengagement from the jaws by having its pair of margins, possibly together with some neighboring parts of the image area, caught tightly between the fixed and the movable jaw. The image area is not engaged so tightly but pressurized only to an extent necessary for creation of a fold on the web section. No ink offset is to occur between the contacting surfaces of the image area as a result of such web section engagement between the fixed and movable jaws.
No ink offset will occur, either, when the folding blade is withdrawn from between the partial folds of the web section, behaving as aforesaid as if prying open the fixed and movable jaws. The withdrawing folding blade will rub hard against the web section in its pair of opposite margins only, or together with some adjoining parts of the image area. The image area of the web section will not be rubbed so hard as to give rise to ink transfer between its contacting surfaces.
Preferably, or as the need arises, projecting pins or like spike means may be provided on those parts of at least either of the fixed and the movable jaw which are to engage the pair of margins of each web section. The web section will then be captured even more positively between the jaws without in any way aggravating the risk of ink offset between its contacting surfaces.
In the practice of the invention the fixed jaw may take the form of a series of spaced-apart fixed jaw parts, and the movable jaw a series of spaced-apart movable jaw parts, the latter being jointly movable toward and away from the respective fixed jaw parts. Out of the series of fixed jaw parts, the two outmost ones (i.e. those at the opposite extreme ends of the series) are to engage at least the pair of margins of each web section, whereas the other, medially situated ones are to engage the image area of each web section. Likewise, the two outmost ones of the series of movable jaw parts are to engage the pair of margins of each web section, and the medial movable jaw parts are to engage the image area of the web section. A less spacing is therefore provided between the outmost fixed jaw parts and the outmost movable jaw parts than between the medial fixed jaw parts and the medial movable jaw parts.
The provision of the different spacings between the outmost fixed and movable jaw parts and between the medial fixed and movable jaw parts is attainable in various ways. In a preferred embodiment the movable jaw parts are fixedly mounted one to each of a plurality of jaw bases which in turn are mounted to a jaw carrier shaft for bidirectional rotation therewith. The fixed jaw parts are all aligned, and the medial movable jaw parts are positioned farther away from the fixed jaw parts than are the outmost movable jaw parts, by mounting the medial movable jaw parts to the jaw bases via spacers. Use of the spacers between the medial movable jaw parts and their jaw bases is preferred because then the jaw bases for all the movable jaws, both outmost and medial, can be of the same construction.
The above and other objects, features and advantages of this invention will become more apparent, and the invention itself will best be understood, from a study of the following description and appended claims, with reference had to the attached drawings showing the preferred embodiment of the invention.